Metal electrodes and coatings thereof

ABSTRACT

An electrode coating having the formula A2BO6, where A represents trivalent elements having anionic radius comprised between 0.50 and 0.73 Angstrom and where B represents hexavalent elements having an ionic radius comprised between 0.50 and 0.70.

United States Patent Degueldre et 211.

Oct. 1, 1974 Assignee: Solvay & Cie, Brussels, Belgium Filed: July 29, 1971 App]. No.: 167,164

Foreign Application Priority Data July 29, 1970 Luxembourg 61433 US. Cl. 204/290 F, 136/120 FC, 204/98, 423/594, 423/595, 423/599, 423/600 Int. Cl 801k 3/06, C01b 11/26 Field of Search 204/290 F, 291; 136/120 FC; 423/593, 594, 599, 600

References Cited UNITED STATES PATENTS Bayer 423/595 Messner 204/290 F Anthony et aL. 204/290 F Bayer 423/593 Sizer, Sr 204/290 F De Witt 204/290 F Langleys 204/290 F Primary Examiner-F. C. Edmundson Attorney, Agent, or FirmSpencer & Kaye ABSTRACT An electrode coating having the formula A BO where A represents trivalent elements having anionic radius comprised between 0.50 and 0.73 Angstrom and where B represents hexavalent elements having an ionic radius comprised between 0.50 and 0.70.

8 Claims, 2 Drawing Figures FIG. I'

FIG.2

BACKGROUND OF THE INVENTION The present invention relates to a new type of coating serving as the operative surface of an electrode.

SUMMARY OF THE INVENTION The present invention concerns a new coating for metal electrodes. The coating comprises a compound having A BO as its general formula. where A represents one or more elements of valence 3, having an ionic radius comprised between 0.50 and 0.73 Angstrom, and B represents one or more elements of valence 6, having an ionic radius comprised, between 0.50 and 0.60 Angstrom. The symbol stands as usual for oxygen.

The coating may be A 80,, alone or an addition compound of A 80,, with another stoichiometric compound having the formula MO or M'M"O,, where M represents one or more elements of valence 4, selected from Ir, Mn, Os, Rh, Ru, Sn and Ti, M represents one or more elements of valence 3 selected from Al, Cr, Fe, Ga and Rh, and M represents one or more elements of valence 5, selected from Nb, Sb, Ta and V.

Preferably, A represents one or more elements selected from Co, Fe, Cr, Mn, Al, Ga, Ir. Rh and V, while B represents one or more elements selected from Te, W, Mo and Re.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring firstly to FIGS. 1 and 2, a conductive coating 11, such as Rh WO is shown placed on a strip of metal 12, such as titanium.

It is preferred to place the conductive coating of the present invention on a metal support at least superficially made of titanium or a titanium meta]. Advantageously, titanium is clad on a core ofa more conductive metal such as copper, aluminum. iron, or alloys of these metals.

Preferably, the coating of the present invention consists, essentially of the compounds as set forth in the appended claims; yet more preferably, the coating consists of those compounds.

Electrodes provided with catalytic coatings according to the invention can be used for different electrochemical processes, such as cathodic protection, desalination or purification of water, water or hydrochloric acid electrolysis, electric current production in fuel cells, reduction or oxydation processes of organic compounds or electrolytic manufacture of peroxide salts but they are particularly useful as anodes for the electrolysis of aqueous solutions of alkali metal halides. especially sodium chloride, in diaphragm cells as well as in mercury-cathode cells. where they catalyse the discharge of chloride ions according to the half reaction 2C1 Cl: 2e under a remarkably low overvoltage which remains substantially unvaried over an electrode lifetime. Under the conditions ruling in these cells, the anode wear is very slow, thus providing practically unlimited lifetime and avoiding the need for cell opening and coating restoration.

Further illustrative of the present invention are the following examples. All concentrations are given on a solution basis, unless indicated otherwise. Thus, for example, a solution of 0.45 g-at W/l means that there are 0.45 gram-atoms of tungsten in one liter of solution. Example 1:

A BO, (alone), where A Rlz and B W.

Dihydrated rhodium nitrate, Rh(NO;,);,.2HO, and tungsten chloride, WCl were separately dissolved at room temperature in N,N-dimethyl formamide (DMF), HCON(CH;,)- to give concentrations of 0.1 gramatoms Rh/liter of solution and 0.46 gram-atoms W/liter of solution, respectively. These two solutions were then mixed in suitable proportions to give a Rh/W atom ratio of 2/1.

Several coats of this composition were painted onto titanium strips which had been degreased in hot trichlorethylene and immersed for 4-5 hours at about 90C in an aqueous solution of 10 percent oxalic acid to etch the surface. For painting, titanium strips were placed on a heating plate at l00-l25C. After each coating, the strips were heated for 15 minutes at from 300 to 450C. After the fifth coating, the coated strips were heated in a furnace at 475C for 5 hours. The coating thus obtained was analyzed and found to contain the compound Rh WO It was present in an amount equal to approximately 5 grams per square meter of the titanium surface. It adhered tightly to the underlying titanium support and passed successfully a test consisting of attempting to tear off the coating with an adhesive tape applied under pressure.

The thus-coated titanium strips were subjected, as anodes, to two different tests: the first one to measure the overvoltage for the liberation of chlorine under a given anodic current density (10 kA/m the second one to determine the wear or consumption of noble metal as related to the quantity of evolved chlorine. The term overvoltage is used herein in the same sense it is used at pages 488-492 ofPhysical Chemistry" by Walter J. Moore, Prentice-Hall Inc., Second Edition.

In the overvoltage test, the coated strips were used as anodes for the electrolysis of a brine containing 250 g NaCl/kg, of solution saturated with chlorine at'60 C and at an approximate pH of 2. Under these conditions, the coated strips of this example showed an overvoltage in the range 170-260 mV under an anodic current density of 10 kA/m In the wear test, the coated strips were used as anodes in a cell with a flowing mercury cathode for the electrolysis of a brine saturated with sodium chloride and chlorine, between and C, under a constant anode-cathode potential difference, the test being stopped when the current density was reduced to one half of its initial value (initial value generally was between 30 and 40 kA/m"). Under these conditions, the tested strips produced 9 tons of chlorine per square meter of active surface; the rhodium consumption lay below 500 mg per ton of chlorine produced under an average current density of 20 kA/m Example 2:

A 80,, (alone), where A Rh and B Te.

The 0.1 gram-atoms/l Rh solution used in Example l was mixed with a solution of TeO in 12 N- hydrochloric acid to give an Rh/Te atom ratio of 2.

Five coats of this composition were applied onto titanium strips under the same conditions as in Example 1. The coating thus obtained was analyzed and found to contain the compound Rh TeO It was present in an amount of approximately g/m and showed a very A 80 (alone), where A Rh and B 1/3 Te 2/3 W.

Hydrated rhodium trichloride and tungsten chloride WCl were separately dissolved in DMF. 0.166 ml of the W solution containing 0.49 g-at W/l and 6.5 mg of tellurium oxide, TeO previously dissolved in 0.1 ml concentrated hydrochloric acid were added to ml of the Rh solution containing 0.0245 g-at Rh/l.

Four coats of this mixture were applied at room temperature onto titanium strips. The strips were dried in air for 30 minutes and heated at 350 C for minutes, after each coating. They were finally heated in a furnace at 500 C for 66 hours.

The thus obtained Rh- (Te W 0,, coating was present in an amount near to 7 g/m' the coating showed an excellent adhesion.

In an overvoltage test carried out under the same conditions as in Example 1, the thus coated strips showed an average overvoltage of 350 mV under a current density of 10 kA/m'-.

Example 4:

A. ,BO; (alone). where A 0.9 Rh +0.1 Fe and B=W.

0.053 ml of a solution of FeCl;,.6H- O in DMF at 0.51 gm Fe/l and 0.28 ml of the tungsten solution of Example 3 were added to 10 ml of the rhodium solution used in Example 3. a. seven coats of this composition were applied at room temperature onto titanium strips. The strips were dried in the air and heated for 15 minutes at 350 C after each application. Upon final firing at 500 C for hours, a coating of 9 g/m (Rh,, ,,Fe,, W0 was obtained which showed a good adhesion. In an overvoltage test carried out under the same conditions as in Example I. the thus coated strips showed an average overvoltage of 290 mV under a current density of 10 kA/m b. The same coating was prepared, but only four coats of the said composition were applied onto the titanium strips disposed on a heating plate. After each coating. the strips were heated in the range 350425 C. They were finally fired at 500 C for 17 hours. The average deposit weight of the thus obtained (Rh Fe hWO coating was 67 g/m The thus coated strips showed better adhesion to the support than strips obtained by applying the same coating composition at room temperature.

In an overvoltage test carried out under the same conditions as in Example I, the thus coated strips Addition compound A BO,;+3MO where A=Cr, B=W

and M=Ru.

Tungsten chloride, WCl.,, CrCl;,.6H O, and hydrated RuCl were dissolved separately in DMF to give solutions of 0.45 g-at W/l, 0.46 gm Cr/l and 0.5 g-at Ru/l repectively, which were mixed in suitable proportions in order to give a Ru/Cr/W atom ratio of 3/2/1.

Five coats of the said composition were applied onto titanium strips under the same Conditions as in Example 1. After firing, a coating of Cr WO +3RuO was obtained. The adhesion of this coating to the support was not perfect.

Nevertheless, used as anodes under the same conditions as in Example 1, the thus coated strips also presented very interesting electrochemical properties. Example 6:

Addition compound A BO 3MO where A Fe, B W and M Ru The 0.51 g-at Fe/l solution of Example 4 and the solutions of 0.46 g-at Cr/l and 0.5 g-at Ru/l of Example 5 were mixed together in suitable proportions to give a Ru/Fe/W atom ratio of 3/2/1.

Five coats of the resulting mixtures were applied onto titanium strips under the same conditions as in Example l, and after firing, a coating of Fe WO +3RuO was obtained. This coating showed relatively poor adhesion to the support,- but interesting anodic polarization properties in chlorinated brine.

Example 7:

Addition compound A 80,; 3MO where A Cr, B

= Te and M Ru. I

The solutions of Cr and Ru mentioned in Example 5 were mixed in a proper proportion and a suitable amount of TeO dissolved in 12 N I-ICl was added in order to obtain a Ru/Cr/Te atom ratio of 3/2/1.

Applied onto titanium strips under the conditions described in Example 1, this composition gave, after firing, a coating of Cr TeO 3RuO whose adhesion to the support was satisfactory and which showed interesting anodic polarization properties in chlorinated brine.

Example 8:

Addition compound A BO. +3MO where A Rh, B Te and M Ru.

A solution A containing 0.2 g-at Ru.l was prepared. First, ruthenium chloride. RuCl .x H O was dissolved in a quantity of ethanol, then chloroform was added in an amount equal to the volume of the ethanol solution and finally turpentine a (boiling point above 150 C), previously sulphurated to 20 percentthrough reflux heating of g turpentine a with 20 g sulphur for 3 hours, was added, also in an amount equal to the volume of the ethanol solution. 1

By operating in the same way with rhodium chloride, RhCl -,.x H O, a solution B containing 0.2 g-at Rh/l was prepared. V

A solution C of 0.2 gm Te/l was prepared by dissolving tellurium chloride, TeCl (resulting from the action of chlorine upon tellurium) in two volumes of ethanol, to which five volumes of turpentine a sulphurated to 40 percent were then added, the whole being heated on a water-bath until the formed precipitate had completely disappeared.

Then, a solution D was prepared by mixing 1 volume of ethanol, one volume of chloroform and 1 volume of turpentine a.

Finally, mixing 3 ml of solution A,

2 ml of solution B,

1 ml of solution C, and

14 ml of solution D,

a mixture was obtained which was applied onto titanium strips previously degreased and etched as described in Example 1.

This coating was carried out in the air at room temperature. After each coating, the strips were heated for 15 minutes at 500 C.

After application of coatings in accordance with the aforesaid conditions, the strips were finally fired at 500C for 5 hours in the presence of air.

The deposit weight of the thus obtained coating of Rh TeO,;+ 3RuO was approximately 4 g/m and its adhesion to the support was acceptable.

In an overvoltage test carried out under the same conditions as indicated in Example 1, the thus coated strips showed an overvoltage of 245 mV under an anodic current density of 10 kA/m In a wear test, also carried out as indicated in Example 1, these coated strips produced 120 tons of chlorine/m working continuously under an average current density of 24 kA/m The consumption of noble metal was under 20 mg Rh Ru per ton of produced chlorme.

Example 9:

Addition compound A 80,; 12 M0 where A Rh, B Te and M Ru.

By dissolving ruthenium chloride RuCl;,.x H O in npentanol. a solution A of one g-at Ru/l was prepared.

By operating in the same way with rhodium chloride RhCl -,.x H O. a solution B of 0.5 g-at Rh/l was prepared.

Then, a solution of allotelluric acid in ethanol of0.5 g-at Te/l (solution C) was prepared. Allotelluric acid was obtained by keeping telluric acid, H- ,TeO, .2H O, at 150C for 90 minutes in a sealed tube.

Mixing 5 ml of solution A.

1.7 ml of solution B,

0.84 ml of solution C, and

41.5 ml of n-pentanol,

a mixture was obtained which was applied onto titanium strips previously degreased and etched as in Example 1.

For the application of this mixture, the titanium strips were disposed. in the air, on a heating plate at approximately 100C. After each application and evaporation of excess solvent, the coated strips were heated for 15 minutes at 350C. After seven coatings had been applied under the same conditions, the coated strips were finally fired for one hour at 500C in the presence of arr.

The thus obtained coating of Rh TeO 12 RuO of about 5 g/m adhered tightly to the support as shown by the test consisting of attempting to tear off the coating with an adhesive tape applied under pressure.

In an overvoltage test carried out as in Example 1, the thus coated strips showed an overvoltage of mV under an anodic current density of 10 kA/m ln a wear test carried out as in Example 1, the coated strips produced more than 82 tons of chlorine/m working continuously under current densities between 36 and 25 kA/m At this stage, the use limit was not reached and the wear test continued.

Example 10:

Addition compound A 80,; 6MO where A Rh. B Te, and M Ir.

A solution of 0.019 g-at lr/l (solution A) was pre pared by dissolving chloro-iridic acid H lrCl,;.xH O in dimethyl sulphoxide (DMSO), CH SOCH A solution ofO. 194 gram-atom Rh/l (solution B) was prepared by dissolving rhodium nitrate Rh(NO;,)- in glycol, HOCH Cl-l Ol-l.

Finally, 25.2 mg T602 were substantially dissolved in a few drops of concentrated HCl (solution C).

Mixing 50 ml of solution A 1.65 ml of solution B and the whole of solution C,

a mixture was obtained which was applied onto titanium strips previously degreased and etched according to Example 1. 1

For the application of the mixture, the titanium strips were disposed, in the air, on a heating plate at about C.

Aftereach application. the coated strips were heated for 5 minutes at 400C.

After 12 coatings had been applied under the same conditions, six further coatings were applied with intermediate heatings of 15 minutes at 440C.

Finally, the coated strips were tired for 16 hours at 475C in the presence of air.

The thus obtained coating of .Rh TeO 6lrO (about 8 g/m passed successfully a test consisting of attempting to tear off the coating with an adhesive tape applied under pressure.

In an overvoltage test carried out as in Example 1,

the thus coated strips showed an overvoltage of mV under an anodic current density of 10 kA/m' In a wear test carried out as in Example 1, these same coated strips produced more than 63 tons of chlorine per square meter of active anodic surface under an average current density of 24 kA/m'-. At this stage, the use limit was not yet reached and the wear test continued.

Example 1 1:

Addition compound 2A BO 3MMO,, where A Rh, B Te, M R1: and M" Sb.

Hydrated RhCl and SbC 1;, were separately dissolved in n-hexanol to give solutions of 0.5 g-at Rh/l (solution A) and 1 g-at Sb/l (solution B) respectively.

5 ml of solution A,

1.1 ml of solution B,

1.4 ml of solution C of Example 9, and

42.5 ml of n-hexanol were mixed.

Seven coatings of the mixture were applied onto titanium strips previously degreased and etched as in Example 1. For the application of the mixture, the strips were disposed on a heating plate at approximately 70C. After each of an initial six applications, the strips were heated for 15 minutes at 500C, whereas after the as claimed in claim 1, wherein the said compound A 80 is rhodium tungstatc, Rh WO 3. A metallic electrode having a conductive coating as claimed in claim 1, wherein the said compound A 80, is rhodium tellurate, Rh TeO 4. A metallic electrode having a conductive coating as claimed in claim 1, said conductive coating further comprising an oxide of M0 type, where M represents at least one element at the valence 4 selected from the group consisting of Ir, Mn, Os, Rh, Ru, Sn, and Ti.

'5. A metallic electrode having a conductive coating as claimed in claim 1, said conductive coating, further comprising anoxide of M'M"0 type, where M repre- Example Composition of Thickness Overvoltage Coating wear test Adhesion in mV Cl Current Consumption No. the coating in g/m at produced density of noble metal kA/m in t/m in kA/m in mg/t produced C1 1 Rh WO 5 excellent 170-260 9 20 500 2 Rh TeO 5 very good 315-430 200 3 Rh. (Te,,;,W. ,,;,)O,; 7 excellent 350 4a (Rh Fe ,WO,, 9 good 290 4b do. 6.7 very good 260-360 5 Cr WO JRuO 5 poor interesting 6 Fe wO jlRuO do. do. 7 Cr TeO,;.3RuO satisfactory do. 8 Rh TeO,,.3RuO 3.7 do. 245 120 24 20 9 Rh TeO .l2RuO 5 very good 95 82 36-25 10 Rh. ,TeO .6 [r0 8 do. 135 63 24 ll 2Rh. ,TeO,,.3RhSbO 2.6 excellent 450 A coating which has interesting overvoltage properties" is a coating for which an overvoltage of about 450 mV may be obtained in the test carried out with chlorinated brine. For industrial uses in electrolysis of NaCl brine for producing chlorine, 500 mV may be considered as the maximum overvoltage acceptable for economic view point. For other electrochemical processes such as cathodic protection and all the uses where Cr and Fe, particularly, are not subject to corrosion, 500 mV is not a critical factor, and so, these coatings may be interesting.

ln the general formula A BO A is an element at the valence 3 having an ionic radius comprised between 0.5 and 0.75 Angstrom and B is an element at the valence 6 having an ionic radius comprised between 0.5 and 0.6 Angstrom. Outside these ionic radius ranges, the compound A 80, of the invention cannot be obtained. According to this restriction the elements may be chosen in the tables published by R. D. Shannon and C. T. Prewitt-Acta. Cryst. 1969, B 25. p. 925 946 and Acta. Cryst.l970, B 26, p. 1046 1048.

Among all these elements, the more commonly available, i.e., Co, Cr, Fe, Mn, Al, Ga, lr, Rh and V for A and Te, W, Mo and Re for B are preferred.

It will be understood that the above description and foregoing Examples of the present invention are susceptible of various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

I. A metallic electrode having a conductive coating, said coating comprising a compound of the general formula A BO wherein A represents at least one element at the valence 3, selected from the group consisting of lr and Rh, and B represents at least one element at the valence 6, selected from the group consisting of Te, W, Mo and Re.

2. A metallic electrode having a conductive coating sents at least one element at the valence 3 selected from the group consisting of Al, Cr, Fe, Ga, and Rh, and M" represents at least one element at the valence 5 selected from the group consisting of Nb, Sb, Ta, and

6. A metallic electrode having a conductive coating as claimed in claim 1 wherein B is at least partially Te.

7. A metallic electrode having a conductive coating,

said coating comprising a compound of the general for mula A wherein A represents at least one element at the valence 3 selected from the group consisting of Co, Cr, Fe, Mn, Al, Ga and V, and B represents at least one element at the valence 6 selected from the group consisting of Te, W, Mo and Re, the said conductive coating further comprising at least one other compound containing at least one element selected from the group consisting of lr, Os, Rh and Ru, said other compound being selected from the group consisting of M0 and M'MO where:

M represents at least one element at the valence 4 selected from the group consisting oflr, Mn, Os, Rh,

Ru, Sn and Ti M represents at least one element at the valence 3 selected from the group consisting of Al, Cr, Fe,.

Ga, and Rh M" represents at least one element at the valence 5 selected from the group consisting of Nb, Sb, Ta and V.

Fe, Mn, Al, Ga and V, and B represents at least one ele-- ment at the valence 6 selected from the group consisting of Te, W, Mo and Re. 

1. A METALLIC ELECTRODE HAVING A CONDUCTIVE COATING, SAID COATING COMPRISING A COMPOUND OF THE GENERAL FORMULA A2BO6, WHEREIN A REPRESENTS AT LEAST ONE ELEMENT AT THE VALENCE 3, SELECTED FROM THE GROUP CONSISTING OF IR AND RH, AND B REPRESENTS AT LEAST ONE ELEMENT AT THE VALENCE 6, SELECTED FROM THE GROUP CONSISTING OF TE, W, MO AND RE.
 2. A metallic electrode having a conductive coating as claimed in claim 1, wherein the said compound A2BO6 is rhodium tungstate, Rh2WO6.
 3. A metallic electrode having a conductive coating as claimed in claim 1, wherein the said compound A2BO6 is rhodium tellurate, Rh2TeO6.
 4. A metallic electrode having a conductive coating as claimed in claim 1, said conductive coating further comprising an oxide of MO2 type, where M represents at least one element at the valence 4 selected from the group consisting of Ir, Mn, Os, Rh, Ru, Sn, and Ti.
 5. A metallic electrode having a conductive coating as claimed in claim 1, said conductive coating, further comprising an oxide of M''M''''04 type, where M'' represents at least one element at the valence 3 selected from the group consisting of Al, Cr, Fe, Ga, and Rh, and M'''' represents at least one element at the valence 5 selected from the group consisting of Nb, Sb, Ta, and V.
 6. A metallic electrode having a conductive coating as claimed in claim 1 wherein B is at least partially Te.
 7. A metallic electrode having a conductive coating, said coating comprising a compound of the general formula A2BO6, wherein A represents at least one element at the valence 3 selected from the group consisting of Co, Cr, Fe, Mn, Al, Ga and V, and B represents at least one element at the valence 6 selected from the group consisting of Te, W, Mo and Re, the said conductive coating further comprising at least one other compound containing at least one element selected from the group consisting of Ir, Os, Rh and Ru, said other compound being selected from the group consisting of MO2 and M''M''''O4 where: M represents at least one element at the valence 4 selected from the group consisting of Ir, Mn, Os, Rh, Ru, Sn and Ti M'' represents at least one element at the valence 3 selected from the group consisting of Al, Cr, Fe, Ga, and Rh M'''' represents at least one element at the valence 5 selected from the group consisting of Nb, Sb, Ta and V.
 8. An electrode suitable for use in electrochemical processes, comprising a support at least superficially containing titanium and having anodic polarization properties comparable with those of titanium, and on at least a part of the surface of said support a conductive coating comprising a compound of general formula A2BO6, wherein A represents one or more elements at valence 3 selected from the group consisting of Co, Cr, Fe, Mn, Al, Ga and V, and B represents at least one element at the valence 6 selected from the group consisting of Te, W, Mo and Re. 